This invention relates generally to temperature controlled heating devices and more particularly to method and apparatus for detecting temperature sensor failure in such devices.
One well known type of temperature controlled heating device is the automatic surface unit used in cooking appliances such as cooktops and ranges. An automatic surface unit is equipped with a temperature sensor for sensing the temperature of the utensil being heated by the surface unit and a controller for controlling the energization of the surface unit as a function of the sensed utensil temperature. Such surface units are well known in the art and typically include a temperature sensing device such as a bimetallic device or a thermistor device mounted to be in thermal contact with the utensil. When the sensed temperature is less than the predetermined threshold temperature set by user manipulation of input control knobs or switches mechanically coupled to the sensing device, the heating unit is energized at full power. When the temperature exceeds the threshold, the unit is de-energized. An electronic control system for an automatic surface unit in which the electromechanical sensing and control devices are replaced by a microprocessor based control arrangement is described in commonly assigned U.S. Pat. No. 4,493,980 which is hereby incorporated by reference.
A problem common to both the electromechanical and electronic temperature sensing arrangements for automatic surface units is that a failure in the sensor circuitry typically results in either the surface unit being operated at full power continuously or being totally de-energized. The user, being unaware of this failure, may be greatly inconvenienced by either overheating or underheating a utensil.
One solution to this problem is disclosed in commonly assigned U.S. Pat. No. 4,639,578. In this arrangement the sensed temperature is periodically compared to a minimum reference temperature lower than normal operating temperature for the lowest operating power level and compared to a maximum reference temperature higher than the normal operating temperature for the highest operating power level. If the sensed temperature is less than the minimum for a period of time greater than a predetermined time to allow for normal transient conditions or higher than the maximum for a predetermined period of time, also to allow for transient conditions, a sensor failure is detected, a user discernible signal indicative of the malfunction of the sensor is generated, and the surface unit is then usable as a standard non-automatic surface unit until the malfunction is corrected. This arrangement reliably detects sensor failures. However, use of this arrangement requires that the analog to digital (A/D) converter circuitry used to convert the sensor signal from analog to digital form for input to the microprocessor programmed to control the surface unit have an operating range which is significantly greater than the range of interest for temperature control. Better temperature control performance can be achieved at less cost using an A/D converter circuit with a range which is more narrowly limited relative to the desired temperature control range. However, actual utensil temperatures, even under normal operating conditions, may be above or below the control range for relatively long periods of time. Thus, when an A/D converter with a range limited to the actual control range is used for temperature sensing, reliance strictly on the temperature signals to detect sensor failures will result in frequent false detects.
Therefore, a need exists for an arrangement which reliably detects temperature sensor failures compatible with control systems in which the range of the temperature sensing system is essentially limited to the control range defined by the maximum and minimum threshold temperatures used for normal temperature control decisions.
It is therefore an object of the present invention to provide an improved sensor fault detection arrangement for use in temperature controlled heating devices in which the range of the temperature sensing system is essentially limited to a control range defined by maximum and minimum threshold temperatures used for normal temperature control decisions. 10 It is a further object of the present invention to provide an improvement to the fault detection arrangement of U.S. Pat. No. 4,639,578, which permits use of a narrower range A/D converter circuit.
It is a further object to provide the foregoing improvement without additional circuitry external to the microprocessor.